Hi all, I'm having 1/2 inch plywood put on the wall studs and then drywall ontop of that for my wetbar to be able to put shelves on the backsplash. the plywood will make the wall strong (so they say) and allow the shelf brackets to be screwed anywhere on the wall rather than having to screw them on the studs. these shelves they will be using only hold about 8 lbs. i'm going to look for my own shelves that hold more weight instead. my question is, will a stud finder detect the wall studs if there is 1/2 inch plywood under the drywall?

Not sure what's typical, but for what you want, I'd use sheathing style plywood in the 3/4" thickness Greg mentioned. OSB won't take screws and anchors as well as plywood. MDF shouldn't be used for structural or weight bearing elements (it's fiberboard, not wood). you don't need hardwood plywood or sanded plywood (even good one side isn't necessary) as it's not going to be visible behind drywall. sheathing style should be cheaper than the others.

I checked home depot for plywood and there are a lot of different kinds. OSB, MDF, hardwood plywood, sanded plywood and sheathing plywood. which of these is generally used in this case?

There are many types of products sold in 4x8 sheets. "Plywood" is just one of those.

OSB - Oriented Strand Board, also called chipboard or Aspenite. It's made of chips of wood glued together. Inexpensive. Often used for sheathing and subflooring. Not super great for holding screws (the chips can break off from each other), and not super resistant to water (chips swell up and break off).

MDF - Medium Density Fibreboard. Basically wood dust glued together under high pressure to form boards. Very heavy. Very smooth surface. Not super great for holding screws (they can rip out and leave a hole that looks like torn cardboard). Not at all resistant to water. Thicker panels used for furniture, thin panels for things like pegboards, slatwall, speaker boxes.

Particle board - Larger wood particles than MDF. Cheap. Heavy. Used for basically all mass-market furniture you buy at IKEA, etc. like desks, cabinets, bookshelves. You can get it plain, or with a thin veneer of something on the outsides so it looks more like real wood. The veneer can be an actual, super-thin, piece of something nicer (like oak) or a "photo of a piece of wood" printed on vinyl and laminated to the outside of the board. "Raw" particleboard is not resistant to water - it will swell up.

Plywood - made of multiple plies of thin sheets of actual wood, glued together with their grains running perpendicular for strength and support. Construction grade plywood is made of spruce, pine, fir (softwood). You can also buy plywood made of hardwoods like maple, oak, birch. Those are used mostly for furniture and cabinetry like a nice oak bureau or a maple bookshelf.

Sanded plywood is a shortcut where they sand one or both sides smooth so you don't need to sand it as much once you've built your furniture or wall and you want to jump right to painting or staining. Not an issue if the plywood will be hidden.

Sheathing plywood - any plywood could technically be used for sheathing, but some are specifically designed with tongues and grooves milled into the edges so they'll interlock with each other when laid on a floor or a wall. This helps ensure there are no gaps and that edges stay together.

All these boards come in varying thicknesses from 1/8" (MDF sheets of pegboard) to 3/4" or more.